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But at least one archaeologist isn't convinced, saying evidence points to climate change playing a key role in their extinction.
Dr Gavin Prideaux of the Western Australian Museum in Perth and colleagues base their conclusion after analysing fossils including the now extinct marsupial 'lion' Thylacoleo, giant wombats, thylacines and giant kangaroos.
"If humans had never come into the country, all of these animals would still be here today," says Prideaux, whose research appears today in the journal Nature.
The well-preserved fossils were discovered in 2002 and are of animals that were trapped in the underground caves in the Nullarbor Plain some 400,000 to 800,000 years ago.
Some researchers argue that such megafauna were wiped out as they couldn't adapt to the arid conditions brought about by ice ages.
But Prideaux and team say humans are more likely to have been the culprits.
They say previous evidence suggests the megafauna died around 40,000-50,000 years ago, at about the time humans arrived.
And now their current study, of a site in southern Australia, provides evidence climate change was unlikely to have played a key role.
Tooth enamel gives clues on past climate
Prideau and team studied oxygen and carbon isotopes in the enamel of kangaroo and wombat teeth found at the site.
They say the analysis suggests the climate at the time was as dry as it is today, suggesting the megafauna were well adapted to dry climates.
But Prideaux says the vegetation must have been very different from that of today's Nullarbor Plain.
"There's no way you could have such a high diversity of herbivores living on a few saltbushes and grasses," he says.
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The now extinct marsupial lion, Thylacoleo (Image: Peter Schouten) |
"What the hell were they doing in the Nullarbor?" he asks. "There had to have been trees."
Prideaux says one possibility is that the megafauna went extinct because of changes to the vegetation due to humans burning it.
"I think it's a good possibility," he says. "That's precisely what happened in Central Australia as well, and it's what's happening in Northern Australia now."
Earlier this month, Prideaux published a paper in the journal Geology that also supports the human activity theory of how the megafauna died.
The paper shows that megafauna from the Naracoorte Caves, 300 kilometres southeast of Adelaide, survived numerous changes in climate over 500,000 years before humans arrived.
"They'd go through the hard times and then recover," he says. "The point is that they disappeared as soon as humans arrived."
A long bow?
Dr Judith Field, an archaeologist from the University of Sydney who favours the climate change theory, says Prideaux and team are drawing "an incredibly long bow".
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Expedition
leader Dr John Long of the Museum Victoria holding a Thylacoleo skull
from one of several complete skeletons recovered form the Nullarbor
Caves (Image: Clay Bryce, Western Australian Museum) |
She says the team implies that all the animals found were around when humans arrived in Australia but evidence suggests only 30% of the megafauna had survived until then.
And Field says Prideaux and colleagues ignore evidence that puts climate change at the scene of the crime.
The past 100,000 years have been drier and more severe than previous periods, she says.
"Humans may well have been involved in the demise of the few remaining species that were here but, as we keep arguing, it's probably a footnote to the broader process that was in train."
Field says Prideaux and team blame human activity without supporting evidence.
Prideaux acknowledges his team has no direct evidence to support the role of humans, but says ruling out climate change supports the theory.
"If you discount one, then the finger swings around and points at the other by default," he says.
Meanwhile Field is frustrated that the Prideaux camp rejects evidence that shows a long period of coexistence between humans and megafauna at Cuddie Springs in New South Wales.
Prideaux's research is funded by the Rio Tinto WA Future Fund.
